Hartig et al. (U.S. Pat. No. 3,770,711) disclose fibers made from copolymers of tetrafluoroethylene (TFE) and 1-7% by weight perfluoropropyl vinyl ether (PPVE). Methyl, ethyl, butyl, and amyl vinyl ether comonomers are also disclosed. Fiber is melt spun with little or no draw-down, followed by a drawing step performed below the melting point. Fibers so fabricated are ca. 500 .mu.m in diameter, exhibiting thermal shrinkage of 15% at 250.degree. C.
Vita et al. (U.S. Pat. No. 5,460,882) disclose multifilament yarns comprising fibers made in a two step process from copolymers of TFE with 2-20 mol % of perfluoroolefins having 3 to 8 carbon atoms, or with 1-5 mol % of perfluorovinylalkyl ethers, the copolymers having a melt flow index of 6-18 g/10 min according to ASTM D3307. In the first step, a fiber is melt spun with a spin stretch factor in the range of 50 to 250, with 50 to 150 preferred; spin stretch factor of 75 spun at 12-18 m/min is exemplified. In the second step, the spun fiber is post-drawn at 200.degree. C. to produce the final product. The as-spun fiber exhibits tenacity of 50 to 80 MPa. In the second step, the as-spun fiber is drawn at a temperature below the melting point to provide a fiber with tensile strength of 140-220 MPa. Fiber diameters of 10 to 150 micrometer diameter (1.7 to 380.times.10.sup.-7 kg/m) are disclosed. Shrinkage of the as-spun fiber at temperatures 40-60.degree. C. below the melting point was 5-10%. The product of the second step drawing process is said to exhibit less than 10% shrinkage at 200.degree. C.
In the process of Umezawa (JP 63-245259), a first step involves forming a mixture of a melt-processible fluorinated resin with a melt-processible hydrocarbon resin wherein the fluorinated resin occupies less than 50% of the volume of the mixture, and forms therein a discontinuous phase dispersed within a continuous hydrocarbon phase. In a second step, a fiber is melt spun from the mixture without draw-down, and in a third step the fiber so formed is drawn below the melting temperature of the fluorinated resin. In a fourth step, the hydrocarbon moiety is dissolved, leaving a very fine linear density fluoropolymer fiber. A TFE/HFP fiber with linear density of 2.2.times.10.sup.-9 kg/m, and tenacity of ca. 400 MPa is exemplified. Disclosed without exemplification is a ca. 3.5.times.10.sup.-8 kg/m fiber of TFE/perfluoroalkoxyethylene with tenacity of 190 MPa.
Nishiyama et. al (JP 63-219616) disclose a process for spinning and drawing fibers from Teflon.RTM. PFA 340-J (Mitsui-DuPont) which retain the cross-sectional shape of the spinneret hole. 110.times.10.sup.-7 kg/m (ca. 80 .mu.m) fiber with 190 MPa tenacity and 17% ultimate elongation is produced by melt spinning without draw-down at 10 m/min, followed by post-drawing 5.times..
Bonigk (P41-31-746 A1- Germany) discloses fiber made from ethylene/tetrafluoroethylene/perfluoropropyl vinyl ether (E/TFE/PPVE) co-polymers wherein the TFE moiety does not exceed 60 mol %. Spinning speed in excess of 800 m/min are disclosed, but spin stretch factor is limited to ca. 100:1. The fibers are characterized by using a thermoplastic copolymer having a melt index of at least 50 g/10 min. (DIN Standard 53 735).
Kronfel'd et al. (Khimicheskie Volokna, No. 1, pp 13-14, 1982) disclose fibers 30-60 micrometer in diameter made by melt spinning a TFE/perfluoroalkyvinyl ether copolymer at a jet stretch of 3500% (corresponding to a spin stretch factor, SSF, of 36) followed by a hot stretch at a ratio of 2.2.times.. The fiber so produced exhibited a tenacity of 14.6 cN/tex (corresponding to ca. 315MPa), a shrinkage in boiling water of 12-15%, and a birefringence of 0.050.
Kronfel'd et al. (Khimicheskie Volokna, No. 2, pp 28-30, 1986) disclose fibers 18 micrometers in diameter and larger of a TFE/perfluoroalkylvinyl ether copolymer containing 3-5 mol % of the vinyl ether. Disclosed is a maximum obtainable spin draw ratio of 850.times. at 400.degree. C. spinning temperature, for polymer of MFR 7.8-18, yielding fiber of maximum tensile strength of 180 MPa.
According to the teachings of the art, which are limited to spin stretch factors of 850.times. or less, usually less than 500.times., low linear density fibers (particularly those of less than 11.times.10.sup.-7 kg/m) can be prepared only by extruding through a narrow extrusion die at low throughputs, at a large economic penalty. Higher extrusion speed, more consistent with low-cost commercial production rates, results in melt fracture and fiber breakage. And, to achieve tensile strengths of greater than ca. 190 MPa requires the additional cost and complexity of a second stage draw on the spun fiber.
Thus, the practices of the known art present several problems to the practitioner thereof. A first problem has to do with producing fiber of linear density below ca. 100.times.10.sup.-7 kg/m, especially less than ca. 40.times.10.sup.-7 kg/m, at commercially practical rates. A second problem has to do with producing fiber with tensile strength of greater than ca. 190 MPa. A third problem has to do with providing for a lower cost process over the slow-speed spinning and multi-step processes of the known art. The fibers produced by the known art also exhibit undesirably high shrinkage of at least 15% at 250.degree. C., limiting their usefulness. Many of the disadvantages of the art are overcome by the process of the present invention wherein the spin stretch factor of the present invention is at least 500. Using the process of the present invention, high strength, low shrinkage low-linear density fibers comprising perfluorinated thermoplastic copolymers of TFE of a wide range of melt flow ratios can be produced at very high spinning speeds in a single step operation, thus increasing productivity and decreasing production costs.